The present invention relates to a method for manufacturing an optical type recording medium and, more particularly, to a method for manufacturing an information recording medium such as an optical disk on which information is written using radiation such as a laser beam.
It is known to preform a track comprising a continuous spiral indentation in a substrate of a recording medium such as an optical disk, and to perform tracking by a servo mechanism in accordance with a difference between reflectances of the track portion of the substrate of the optical disk and the remaining flat portion thereof. In the case of writing data on or reading it out from a conventional optical disk having a track comprising a continuous spiral indentation (also called a "pregroove"), the tracking control technique of the type described above is very effective in accurately tracing the track with an optical head. In this case, information or data is stored in the form of holes called recording pits in a recording layer. These recording pits are formed by focusing the recording laser beam on the pregroove of the optical disk of the type described above.
The shape and size of the pregroove of the optical disc are properly determined so as to increase the precision of tracking control. In order to achieve high tracking control precision, it is necessary to considerably increase the level of an error signal generated when the laser beam (write or read beam) spot is undesirably deviated from the pregroove. Furthermore, the shape and size of the pregroove of the optical disk are also determined so as to efficiently provide reflected light which indicates the presence or absence of recording pits on the optical disk, in order to increase the level of a data read signal from the optical disk along the pregroove thereof. Therefore, it is preferred that the pregroove in the optical disk be formed uniformly over the entire area of the optical disk so as to have a groove width of not more than 1 .mu.m and a groove depth of about 1/8 the wavelength of the laser beam used (e.g., about 0.1 .mu.m).
Conventionally, an optical disk with a continuous spiral pregroove or track is manufactured as follows. A chromium film is formed on a top surface of a transparent substrate comprising disk-shaped glass (of, e.g., 300 mm diameter). A photoresist material is applied by a spinner or the like to the surface of the chromium film, thus forming a photoresist film. The substrate having the photoresist film formed thereon in a manner described above is then rotated at a predetermined speed. The laser beam is then focused to irradiate the photoresist film of the rotating substrate so as to form a beam spot of 1 .mu.m in diameter. This laser beam is moved at a constant feed speed along the radial direction of the rotating disk-shaped substrate. When irradiation by the laser beam is completed, the structure exposed to the laser beam is etched, thereby obtaining an original disk which has a continuous spiral pregroove. The track may alternatively comprise a projection or ridge, depending on the type of photoresist material. Electrodes are deposited on the original disk, and an obtained structure is subjected to electroforming. A disk structure having a transferred indentation of the original disk is separated therefrom to form a stamper. Then, a desired resin disk is obtained using the stamper, and thus, a final optical disk is manufactured.
However, in the conventional method described above, it is very difficult to uniformly form the spiral track portion to have a width of 1 .mu.m and a depth of 0.1 .mu.m over the entire surface of the disk-shaped substrate of 300 mm in diameter. In general, when the photoresist material is applied to the substrate to form a photoresist film having a thickness on the order of microns, the photoresist film tends to partially peel off from the disk-shaped substrate. As a result, uniformity of the film cannot be expected. In particular, when the photoresist film is formed to have a thickness on the order of submicrons, the photoresist material is diluted before being applied to the surface of the substrate. As a result, the uniformity of the photoresist film is further degraded, and irregular development and etching occur. Nonuniformity occurring at the beginning of the disk manufacturing process results in a definite fault in the optical disk. In this manner, a spiral track having the dimensions described above over the entire surface of the substrate of the optical disk is extremely difficult to form.